The demand for various wideband telecommunications services such as high resolution video, voice communications, and terminal and computer connections provides an impetus for the introduction of a broadband digital trunk and exchange network. One example of a broadband digital trunk and exchange network is known as the Broadband Integrated Digital Services Network (B-ISDN) (see e.g. L. R. Linnell, "A Wide-Band Local Access System Using Emerging-Technology Components," IEEE Journal on Selected Areas in Communications, vol. 4, no. 4, pp. 612-618, July 1986; D. R. Spears, "Broadband ISDN Switching Capabilities From a Services Perspective," IEEE Journal on Selected Areas in Communications, vol. 5, no. 8, pp. 1222-1230, October 1987).
Many network issues such as packet transmission and multiplexing techniques (see e.g., H. J. Chao, "Design of Transmission and Multiplexing System For Broadband Packet Networks," IEEE Journal on Selected Areas in Communications, vol. 6, no. 9, pp. 1511-1520, December 1988), packet switching network designs (see e.g. C. Day, J. Giacopelli, and J. Hickey, "Application of Self-Routing Switches to LATA Fiber Optic Networks," in Proc. 1987 International Switching Symposium, Phoenix, Ariz., March 1987; T. T. Lee, "Nonblocking Copy Networks for Multicast Packet Switching," IEEE Journal on Selected Areas in Communications, vol. 6, no. 9, pp. 1455-1467, December 1988), and routing and flow control (see e.g. J. Y. Hui, "Resource Allocation for Broadband Network," IEEE Journal on Selected Areas in Communications, vol. 6, no. 9, pp. 1598-1608, December 1988) have been widely explored for broadband digital trunk and exchange networks such as B-ISDN.
To provide broadband telecommunications services, along with a broadband digital trunk and exchange network, a customer premises network is also utilized. The customer premises network provides interface connections between a broadband digital trunk and exchange network and individual units of customer premises equipment. The customer premises equipment includes devices such as telephones, fax machines, video display devices, and various types of computer terminals and workstations.
Information transmitted across a broadband digital trunk and exchange network such as B-ISDN may be carried using the Asynchronous Transfer Mode (ATM) technique (see e.g. S. E. Minzer, "Toward an International Broadband ISDN Standard" Telecommunications, October 1987 and Chao et al, U.S. Pat. application Ser. No. 118,977 filed on Nov. 10, 1987, now U.S. Pat. No. 4,893,306, issued Jan. 9, 1990, and assigned to the assignee hereof). In the ATM technique, data from various services are transmitted in fixed length cells, which cells are embedded in the payload envelopes of the frames which comprise the Synchronous Optical Network (SONET) STS-3c or STS-12c signals (see.e.g. Draft of American National Standard For Telecommunications Digital Hierarchy Optical Interface Rates and Formats Specifications, T1X1.4/87-505 R4, December 1987). The technique is called the Asynchronous Transfer Mode because the individual cells are not synchronously reserved for specific services, but instead individual cells are occupied dynamically by the specific services based on the availability of cells and the demand for transmission capacity by the specific services.
Typically, a customer premises network receives from a broadband digital trunk and exchange network an ATM type bit stream whose cells contain data from a plurality of different services and are destined to a plurality of different units of customer premises equipment. The customer premises network serves to direct the data from each specific service to the appropriate unit of customer premises. The data for each specific unit of customer premises equipment does not arrive at predetermined times and may be bursty. From this arriving data, each specific unit of customer premises equipment recovers a clock frequency for the particular service it is receiving.
Conventional clock recovery circuits (see Floyd M. Gardner, "Phaselock Techniques", John Wiley & Sons, New York, 1979; and D. K. Jeong et al, "Design of PLL-Based Clock Generation Circuits", IEEE Journal of Solid State Circuits, Vol. SC-22, No. 2, April 1987) are not suitable for this purpose. The conventional clock recovery circuits make use of constant incoming data which embeds timing information perfectly synchronized with some master clock generator. Services with information rates different from the master clock are bit-stuffed and then transmitted. Timing information is extracted to regenerate the transmission clock by using a phase-locked loop.
In situations where the information does not arrive at predetermined times and where the information is bursty (such as in a network using ATM), the above described conventional clock recovery circuits are not suitable to recover the clock. Accordingly, it is an object of the present invention to provide a clock recovery circuit which can recover a clock from information which does not arrive at predetermined times and which is bursty. It is a further object of the present invention, to provide a clock recovery circuit for use in a customer premises network using the ATM transmission technique to enable each specific unit of customer premises equipment attached to the customer premises network to recover a service clock from the data of the specific service destined to it.